Coated article and method of making

ABSTRACT

An article includes a silicon-containing substrate and a modified mullite coating. The modified mullite coating comprises mullite and a modifier component that reduces cracks in the modified mullite coating. The article can further comprise a thermal barrier coating applied to the modified mullite coating. The modified mullite coating functions as a bond coating between the external environmental/thermal barrier coating and the silicon-containing substrate. In a method of forming an article, a silicon-containing substrate is formed and a modified mullite coating is applied. The modified mullite coating comprises mullite and a modifier component that reduces cracks in the modified mullite coating.

[0001] This invention was made with government support under ContractNo. NAS3-26385 awarded by NASA. The government may have certain rightsin the invention.

BACKGROUND OF THE INVENTION

[0002] The invention relates to an article having at least a modifiedmullite coating. The invention further relates to a silicon-containingsubstrate having at least a modified mullite coating. The inventionfurther relates to a silicon-containing ceramic substrate having amodified mullite coating and at least one additional layer of material.

[0003] Silicon-containing materials have been proposed for structuresused in high temperature applications, such as in heat exchangers andadvanced internal combustion engines. For example, silicon-basedcomposite ceramics have been proposed as materials for applications incombustors for commercial airplanes. However, these ceramic materialsexhibit poor oxidation resistance in reducing atmospheres and inenvironments containing salts, water vapor or hydrogen. Hence, it isnecessary to apply environmental barrier coatings to thesilicon-containing materials to provide protection from environmentalattack at elevated temperatures and to apply thermal barrier coatings toextend the life at elevated temperatures.

[0004] Mullite has been proposed as a material for environmental barriercoatings as well as thermal barrier coatings on silicon-containingmaterials. Mullite exhibits low thermal conductivity. It has low densityand a high melting point. However, mullite coatings tend to developcracks perpendicular to substrates and through the thickness of thecoating. These cracks are detrimental to the functions of the mullitecoating because they serve as transport paths for corrosive speciescausing severe oxidation and corrosion at the interface between thecoating and substrate. Additionally, cracks in the coating concentratestresses. The cracks apply shear and tensile forces on the substrate tocause substrate fractures.

[0005] Since the crack openings increase with increasing distance fromthe mullite substrate interface, the cracks may be the result of thedifference in thermal expansion between the mullite coating and thesilicon-containing substrate. FIG. 1 shows differences in thecoefficient of thermal expansion (CTE) of mullite, silicon carbide (SiC)and silicon (Si). Thus, there is a need to provide coatings or layers tosilicon-containing substrates that act at least as environmental barriercoatings having reduced cracks.

SUMMARY OF THE INVENTION

[0006] The present invention is based on the discovery that a modifiercomponent can be added to a mullite coating to reduce cracks in thecoating applied to a silicon-containing substrate. The mullite coatingwith the modifier component is also referred to as a modified mullitecoating. The modified mullite coating reduces fracture at the interfaceof the mullite coating and the silicon-containing substrate.

[0007] In one aspect, the invention is an article comprising asilicon-containing substrate and a modified mullite coating. Themodified mullite coating comprises mullite and a modifier component thatreduces cracks, including through-thickness cracks, in the mullitecoating. Preferably, the modifier component comprises a component havinga lower thermal expansion than the mullite coating. As a result, themodifier component imparts a lower thermal expansion coefficient to themullite coating. The article can further comprise an externalenvironmental/thermal barrier coating applied to the modified mullitecoating. The modified mullite coating then functions as a bond coatbetween the external environmental/thermal barrier coating and thesilicon-containing substrate.

[0008] In another aspect, the invention relates to a method of formingan article with at least a modified mullite coating. In the method, asilicon-containing substrate is formed and a modified mullite coating isapplied. The modified mullite coating comprises mullite and a modifiercomponent that reduces cracks in the coating.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a graph illustrating comparative coefficient of thermalexpansions for mullite, silicon carbide and silicon.

[0010]FIG. 2 is a graph illustrating comparative coefficient of thermalexpansions for mullite, cordierite, fused silica and celsian(BaO.Al₂O₃.2SiO₂).

[0011]FIG. 3 is a photomicrograph for a mullite and yttria stabilizedzirconia-coated silicon carbide/silicon carbide composite.

[0012]FIG. 4 is a photomicrograph for a mullite with twenty-two volumepercent calcium aluminosilicate and a yttria stabilized zirconia-coatedsilicon carbide/silicon carbide composite; and

[0013]FIG. 5 is a photomicrograph for a mullite with eighteen volumepercent barium strontium aluminosilicate and a yttria stabilizedzirconia-coated silicon carbide/silicon carbide composite.

DETAILED DESCRIPTION OF THE INVENTION

[0014] According to the invention, a modifier component is added to amullite coating to reduce or eliminate cracks, includingthrough-thickness cracks. By through thickness-cracks is meant cracksthat extend substantially through the entire thickness of the mullitecoating from near the top surface to near the bottom of the coating ornear the silicon-containing substrate. The modifier components can becategorized into one or more of at least three functional groups. (1)The modifier component imparts a closer coefficient of thermal expansion(CTE) match between the modified mullite coating and silicon-containingsubstrate than the coefficient of thermal expansion match between themullite coating without the modifier and the silicon-containingsubstrate. (2) The modifier component provides a phase or phases thatreduce the overall elastic modulus of the modified mullite coating toreduce thermal stress in said coating. (3) The modifier componentprovides a phase or phases that serve as crack arresters to increaseresistance of the modified mullite coating to crack propagation. Themodifer component increases the toughness of the modified mullitecoating.

[0015] The modified mullite coating is applied to a silicon-containingsubstrate. Suitable silicon-containing substrates comprise materialsthat result in cracking of an applied mullite coating. Thesilicon-containing substrate can comprise a ceramic such as asilicon-based ceramic. Examples are silicon carbide, silicon nitride,silicon carbon nitride, silicon oxynitride, and the like. Thesilicon-containing ceramic substrate can be a monolith or composite. Acomposite can comprise a silicon, silicon carbide, carbon or mixturesthereof reinforcing fibers, particulate or whiskers and a silicon-basedmatrix. The fibers, particulate and whiskers generally have at least oneouter coating, such as silicon carbide, silicon boride, silicon nitrideand the like. The matrix can be processed by melt infiltration (MI),chemical vapor infiltration (CVI) or other technique. Exemplarysilicon-containing substrates include a monolithic silicon carbide andsilicon nitride substrate, a silicon carbide fiber-reinforced siliconcarbide matrix composite, carbon fiber-reinforced silicon carbide matrixcomposite, and a silicon carbide fiber-reinforced silicon nitridecomposite. The preferred substrate comprises a silicon carbidefiber-reinforced silicon-silicon carbide matrix composite processed bysilicon melt infiltration.

[0016] Also suitable as silicon-containing substrates are silicon metalalloys. These alloys include niobium silicon alloys, molybdenum siliconalloys and the like.

[0017] The coated article of the invention can comprise a thermalbarrier coating applied to the modified mullite coating. Suitableexternal environmental/thermal barrier coatings include chemicallystabilized zirconias, such as yttria-stabilized zirconia,scandia-stabilized zirconia, calcia-stabilized zirconia andmagnesia-stabilized zirconia, alumina and alumina silicate. Preferredexternal environmental/thermal barrier coatings in this inventioninclude yttria-stabilized zirconia (YSZ), barium strontiumaluminosilicate (BSAS), calcium aluminosilicate (CAS) and yttriumsilicates (YS).

[0018] The modified mullite coating of the invention comprises mulliteand a modifier component that reduces cracks in the modified mullitecoating. Mullite is a stable form of aluminum silicate found naturallyor formed by heating other aluminum silicates such as cyanite,sillimanite and andalusite, to high temperature. Mullite is an excellenthigh temperature material (incongruent melting temperature about 1830C.) with high corrosion resistance, high thermal shock resistance andchemical stability at high temperatures, such as up to about 1700 C. Itis the only stable crystalline compound in the aluminum silicate systemunder normal atmospheric pressure. It has a chemical composition rangingfrom 3Al₂O₃—2SiO₂ (71.8 wt % Al₂O₃) (3/2 mullite) to approximately2Al₂O₃—SiO₂)₂ (77.3 wt % Al₂O₃)(2/1 mullite). It crystallizes in theorthorhombic system. It has a melting point of 1850 C. and a coefficientof thermal expansion of 5.62×10⁻⁶/C. in the 25-1500 C. range. In theabsence of glassy inclusions, mullite retains greater than 90% of itsroom temperature strength to 1500 C. and displays very high creep andthermal shock resistance.

[0019] Examples of suitable modifier components of the modified mullitecoating include alkaline earth aluminosilicates, preferably with theformula MO.Al₂O₃.2SiO₂, where M is an alkaline earth element. Preferredmodifier components of the formula MO.Al₂O₃.2SiO₂ include bariumfeldspar (BaO.Al₂O₃.2SiO₂), strontium feldspar (SrO.Al₂O₃.2SiO₂), andcombinations of barium feldspar (BaO.Al₂O₃.2SiO₂), and strontiumfeldspar (SrO.Al₂O₃.2SiO₂). Preferably, the alkaline earthaluminosilicate has a monoclinic celsian crystalline phase. Mostpreferred aluminosilicates include (BaO)_(0.75)(SrO)₀ ₂₅.Al₂O₃.2SiO₂referred to as BSAS, CaO.Al₂O₃.2SiO₂ referred to as CAS andBaO.Al₂O₃.2SiO₂. Other suitable modifiers include materials referred toas NZP's such as NaZr₂P₃O₁₂, Ba₁ ₂₅Zr₄P₅ ₅Si₀ ₅O₂₄, Ca₀ ₅Sr₀ ₅Zr₄(PO₄)₆and Ca₀ ₆Mg_(0.4)Sr₄(PO₄)₆. Other preferred modifier components includeyttrium silicates, calcium aluminates including 3Ca₀ ₅.5Al₂O₃, aluminumtitanates including Al₂O₃.TiO₃, cordierite (2MgO.Al₂O₃.5 SiO₂), fusedsilica (SiO₂) and silicon (Si). These materials are also chemicallycompatible with mullite.

[0020] The modifier components may be added to the modified mullitecoating in a percent volume range between about 5 to about 50.Preferably, the modifier component is present in about 10 to about 30volume percent of the modified mullite coating and most preferably inabout 15 to 25 volume percent.

[0021] In the group (1) modifier components, a coefficient of thermalexpansion is imparted to the modified mullite coating that is closer tothe coefficient of thermal expansion of the silicon-containingsubstrate. The coefficient of thermal expansion of a polycrystallinecomposite material is determined by the volume fractions of itsconstituents. The coefficient of thermal expansion can generally beapproximated by using the rule of mixture:

α_(c)=α₁ V ₁+α₂ V ₂+ . . . α_(i) V _(i)

[0022] where α_(c) is the coefficient of thermal expansion of thecomposite, and α₁, α₂ and α_(i) and V₁, V₂ and V_(i) are the coefficientof thermal expansions and volume fractions of phases 1, 2 and i,respectively. Therefore, adding a phase or phases with lower coefficientof thermal expansion to a material will result in a composition that hasa lower coefficient of thermal expansion than the starting material. Toduplicate the coefficient of thermal expansion of the silicon-containingsubstrate, the volume faction of the modifier component in the modifiedmullite coating should be proportionate to the ratio of the differencebetween the coefficient of thermal expansion of the silicon-containingsubstrate and the coefficient of thermal expansion of the mullite to thedifference between the coefficient of thermal expansion of the modifiercomponent and the coefficient of thermal expansion of the mullite.

[0023] A comparison of coefficient of thermal expansion of mullite withthe coefficient of thermal expansions of cordierite, fused silica andcelsian (BaO.Al₂O₃.2SiO₂) is illustrated in FIG. 2. According to theinvention, co-depositing mullite with a low thermal expansion modifiercomponent such as cordierite, fused silica or celsian (BaO.Al₂O₃.2SiO₂)on silicon or silicon-containing ceramic substrates or ceramic compositesubstrates imparts an improved thermal expansion match of the modifiedmullite coating with the silicon-containing substrate than with amonolithic mullite coating.

[0024] Cordierite is an incongruently melting compound with mulliteformed first when cooling from the liquid phase. Upon quenching form themelt splash during the plasma spray, it may remain as a glassy materialor mullite with a glass phase. This may require a post-spray annealingprocess at appropriate temperatures to convert the material tocordierite. The amount of modifier component addition can be firstestimated by the rule of mixture estimate. But because of the complexityof phase composition in the system, a trial and error process may haveto be executed before an optimal proportion is reached.

[0025] The mullite coating with the modifier component can be applied tothe silicon-containing substrate by any suitable method includingthermal spray, air plasma spray (APS) and vacuum or low pressure plasmaspray (VPS or LPPS), high velocity oxy-fuel (HVOF) spray, vapordeposition, including chemical vapor deposition (CVD), physical vapordeposition (PVD) and solution techniques such as sol-gel, slurry coatingor colloidal suspension coating. A constituent starting powder of themullite coating and modifier component may be premixed through avigorous mechanical process, such as ball milling, to provideinterlocking of the powders and prevent segregation of phases due todensity differences. For the same purpose, a sol-gel or colloidalprocess may be employed to coat the particles of one constituent withanother.

[0026] Sarin et al., U.S. Pat. No. 5,763,008 and Lee et al., U.S. Pat.No. 5,496,644 describe exemplary methods of applying mullite coatings.The disclosures of these patents are incorporated herein by reference.Sarin et al discloses a chemical deposition process comprising steps ofestablishing a flow of reactants which will yield mullite in a CVDreactor, and depositing a crystalline coating from the reactant flow.Lee et al. discloses a method of plasma spraying mullite coatings ontosilicon-based ceramic materials. The method prevents deposition ofamorphous mullite by heating the silicon-containing substrate to a veryhigh temperature (greater than 1000 C.) during the spraying process.

[0027] The following examples are illustrative of the invention.

EXAMPLES

[0028] Powders of CaO.Al₂O₃.2SiO₂ (CAS) (22 vol %) and (BaO)₀ ₇₅(SrO)₀₂₅.Al₂O₃.2SiO₂ (BSAS) (18 vol %) were added to mullite powder by ballmilling, respectively. The composite powders were sprayed using airplasma spray (APS) onto a silicon carbon fiber reinforced siliconcarbide-silicon matrix composite substrate processed by meltinfiltration. The substrate temperature was kept at 1050 to 1250 C. Theplasma torch model was Electro-plasma 03CA, with 45 kW power, argon(14.4 SLM) as primary gas and helium (9.8 SLM) as secondary gas. Plasmatorch to substrate distance was 4″. A top coat of yttria-stabilizedzirconia (YSZ) was applied on top of the composite mullite coating byair plasma spray using standard operating procedures for thermal barriercoatings. A baseline sample of monolithic mullite coating on the ceramiccomposite substrate was also prepared using the thermal spray techniquewith a yttria-stabilized zirconia topcoat.

[0029] Samples of silicon-containing ceramic substrates with themodified mullite and thermal barrier coatings and monolithic mullitecoatings were subjected to an environmental furnace test with two hourcycles from room temperature to 1300 C. for 500 hours in 90% H₂O 10% O₂.The results are shown in FIGS. 3-5.

[0030]FIG. 3 shows that through-thickness cracks developed in thebaseline sample with the monolithic mullite coating. Extensive oxidationof the silicon-based ceramic composite at the mullite/substrateinterface resulted in failure of the mullite coating (environmentalbarrier coating) during the test.

[0031] In contrast, the composite modified mullite coatings shown inFIGS. 4 and 5 exhibited no through-thickness cracks in the modifiedmullite coating and the coatings survived the test with minimal changeat the modified mullite coating/substrate interface.

What is claimed:
 1. An article comprising: a silicon-containingsubstrate; and a modified mullite coating comprising mullite and amodifier component that reduces cracks in the modified mullite coating.2. The article of claim 1, wherein said modifier component comprises alower thermal expansion component than mullite that imparts a closercoefficient of thermal expansion match between said modified mullitecoating and said silicon-containing substrate.
 3. The article of claim1, wherein said modifier component forms a phase in said modifiedmullite coating that reduces thermal stress in the modified mullitecoating by reducing the elastic modulus of said modified mullitecoating.
 4. The article of claim 1, wherein said modifier componentforms a phase in said modified mullite coating that arrests thepropagation of cracks in said modified mullite coating.
 5. The articleof claim 1, wherein said modifier component comprises an alkaline earthaluminosilicate.
 6. The article of claim 1, wherein said modifiercomponent comprises a modifier component of the formula MO.Al₂O₃.xSiO₂,where M is an alkaline earth element and 1≦x≦3.
 7. The article of claim1, wherein said modifier component comprises barium feldspar(BaO.Al₂O₃.2SiO₂), strontium feldspar (SrO.Al₂O₃.2SiO₂) or a combinationof barium feldspar (BaO.Al₂O₃.2SiO₂), and strontium feldspar(SrO.Al₂O₃.2SiO₂).
 8. The article of claim 1, wherein said modifiercomponent comprises a monoclinic celsian crystalline phase.
 9. Thearticle of claim 1, wherein said modifier component comprises bariumstrontium aluminosilicate (BSAS), calcium aluminosilicate (CAS), yttriumsilicate (YS) or a combination thereof.
 10. The article of claim 1,wherein said modifier component comprises an NZP (NaZr₂P₃O₁₂) modifier.11. The article of claim 10, wherein said modifier component comprisesNaZr₂P₃O₁₂, Ba₁ ₂₅Zr₄P₅ ₅SiO₀ ₅O₂₄, Ca₀ ₅Sr₀ ₅Zr₄(PO₄)₆ or Ca₀ ₆Mg₀₄Sr₄(PO₄)₆.
 12. The article of claim 1, wherein said modifier componentcomprises a calcium aluminate or an aluminum titanate.
 13. The articleof claim 1, wherein said modifier component comprises 3Ca0.5.5 Al₂O₃, orAl₂O₃.TiO₂.
 14. The article of claim 1, wherein said modifier componentcomprises cordierite (2MgO.2Al₂O₃.5SiO₂), fused silica (SiO₂) silicon(Si), or mixture thereof.
 15. The article of claim 1, wherein saidsilicon-containing substrate is a monolithic or composite siliconcarbide/silicon ceramic.
 16. The article of claim 1, wherein saidsilicon-containing substrate is a monolithic or composite siliconnitride.
 17. The article of claim 1, further comprising an externalenvironmental/thermal barrier coating applied to said modified mullitecoating.
 18. The article of claim 17, wherein said modified mullitecoating functions as a bond coating between said externalenvironmental/thermal barrier coating and said silicon-containingsubstrate.
 19. The article of claim 17, wherein said thermal barriercoating comprises yttria-stabilized zirconia, scandia-stabilizedzirconia, calcia-stabilized zirconia, magnesia-stabilized zirconia,alumina or alumina silicate.
 20. The article of claim 2, wherein avolume faction of the modifier component in the modified mullite coatingis proportionate to a ratio of a difference between a coefficient ofthermal expansion of the silicon-containing substrate and thecoefficient of thermal expansion of a mullite to a difference between acoefficient of thermal expansion of the modifier component and thecoefficient of thermal expansion of the mullite.
 21. A method of formingan article, comprising; forming a silicon-containing substrate; andapplying a modified mullite coating comprising mullite and a modifiercomponent that reduces cracks in the modified mullite coating.
 22. Themethod of claim 21, further comprising annealing said modified mullitecoating to convert a glass phase of said modifier component to a phasecompatible with said mullite.
 23. The method of claim 21, furthercomprising applying an external environmental/thermal barrier coatingonto said modified mullite coating.
 24. The method of claim 21, whereinsaid modifier component comprises a lower thermal expansion componentthan mullite that imparts a closer coefficient of thermal expansionmatch between said modified mullite coating and said silicon-containingsubstrate.
 25. The method of claim 21, wherein said modifier componentforms a phase in said modified mullite coating that reduces thermalstress in the modified mullite coating by reducing the elastic modulusof said modified mullite coating.
 26. The method of claim 21, whereinsaid modifier component forms a phase in said modified mullite coatingthat arrests the propagation of cracks in said modified mullite coating.27. The method of claim 21, wherein said modifier component comprisesalkaline earth aluminosilicates.
 28. The method of claim 21, whereinsaid modifier component comprises a modifier component of the formulaMO.AL₂O₃.2SiO₂, where M is an alkaline earth element.
 29. The method ofclaim 21, wherein said modifier component comprises barium feldspar(BaO.Al₂O₃.2SiO₂), strontium feldspar (SrO.Al₂O₃.2SiO₂) or a combinationof barium feldspar (BaO.Al₂O₃.2SiO₂), and strontium feldspar(SrO.Al₂O₃.2SiO₂).
 30. The method of claim 21, wherein said modifiercomponent comprises a monoclinic celsian crystalline phase.
 31. Themethod of claim 21, wherein said modifier component comprises bariumstrontium aluminosilicate (BSAS), calcium aluminosilicate (CAS), yttriumsilicate (YS) or a combination thereof.
 32. The method of claim 21,wherein said modifier component comprises an NZP modifier.
 33. Themethod of claim 32, wherein said modifier component comprisesNaZr₂P₃O₁₂, Ba₁ ₂₅Zr₄P₅ ₅Si₀ ₅O₂₄, Ca₀ ₅Sr_(0.5)Zr₄(PO₄)₆ or Ca₀ ₆Mg₀₄Sr₄(PO₄)₆.
 34. The method of claim 21, wherein said modifier componentcomprises a calcium aluminate or an aluminum titanate.
 35. The method ofclaim 21, wherein said modifier component comprises 3Ca0.5.5 Al₂O₃, orAl₂O₃.TiO₃.
 36. The method of claim 21, wherein said modifier componentcomprises cordierite (2MgO.2Al₂O₃.5SiO₂), fused silica (SiO₂) or silicon(Si).
 37. The method of claim 21, comprising adding a volume faction ofthe modifier component in the modified mullite coating is proportionateto a ratio of a difference between a coefficient of thermal expansion ofthe silicon-containing substrate and the coefficient of thermalexpansion of a mullite to a difference between a coefficient of thermalexpansion of the modifier component and the coefficient of thermalexpansion of the mullite.
 38. The method of claim 21, comprisingapplying said modified mullite coating by thermal spray, high velocityoxy-fuel (HVOF) spray, vapor deposition, physical vapor deposition (PVD)or solution technique.
 39. The method of claim 21, comprising applyingsaid modified mullite coating by air plasma spray, vacuum plasma sprayor low pressure plasma spray.
 40. The method of claim 21, comprisingapplying said modified mullite coating by chemical vapor deposition. 41.The method of claim 21, comprising applying said modified mullitecoating by sol-gel, slurry coating or colloidal suspension coating. 42.The method of claim 21, comprising forming the modified mullite coatingby first forming a starting powder of the coating and modifier componentby ball milling.
 43. The method of claim 21, comprising forming themodified mullite coating by a sol-gel or colloidal process.
 44. A methodfor making a coated article comprising forming a silicon/silicon carbidecomposite having silicon carbide-containing fibers; applying a modifiedmullite coating comprising mullite and barium strontium aluminosilicate(BSAS), that reduces cracks in the modified mullite coating; andoptionally applying a yttria-stabilized zirconia coating to the modifiedmullite coating.
 45. A coated article comprising a silicon/siliconcarbide composite having silicon carbide-containing fibers; a modifiedmullite coating comprising mullite and barium strontium aluminosilicate(BSAS), that reduces cracks in the modified mullite coating; andoptionally a yttria-stabilized zirconia coating on the modified mullitecoating.